1. Field of the Invention
The present invention relates to a photomask used for a projection exposure equipment and more particularly, to a photomask used to form a fine pattern in a semiconductor device fabrication process.
2. Description of the Related Art
Recently, optical lithography has been a mainstay in forming a pattern on a semiconductor substrate in a semiconductor device fabrication process.
In an optical lithographic process, first a pattern formed on a photomask is transferred to photosensitive resin coated on a semiconductor substrate surface by a reduction projection exposure equipment. The photosensitive resin is patterned to a predetermined pattern in development. A pattern formed on a photomask comprises a transparent region and a light-intercepting region. A photomask is an original plate for exposure on which such a pattern is formed and it is called a reticle when a size ratio between patterns formed thereon and on photosensitive resin is not 1:1.
Miniaturization of a pattern for a semiconductor device has recently been demanded and in a conventional photolithographic technology, such a demand has been satisfied with development of an exposure equipment, especially with a higher numerical aperture (NA) in a projection lens system. Numerical aperture is an index showing a power to collect spread optical rays and the higher the value, the higher the performance of a lens since more spread optical rays can be collected.
The following mathematical formula (1) is established between the limit R of a resolution, that is the minimum dimension of a fine pattern with which images can be separated, and a numerical aperture NA, as a Rayleigh formula, which is: EQU R=(K.sub.1 .times..lambda.)/NA (1)
where K.sub.1 is a constant which depends on process parameters, such as a performance of photosensitive resin and the like, and k is wavelength of transmitted light. As can be seen from the formula (1), as a value of a numerical aperture NA is larger, the resolution limit R is smaller.
A resolution can be improved by increasing a numerical aperture of an exposure equipment but since a depth of focus is decreased with the increase in resolution, there arises difficulty in miniaturization of a pattern. A depth of focus is an index showing an allowable range of a displacement of a focal point. The following mathematical formula (2) is established between a depth of focus DOF and a numerical aperture NA, as a Rayleigh formula, which is: EQU DOF=(K.sub.2 .times..lambda.)/NA.sup.2 (2)
where K.sub.2 is a constant depending on process parameters. As can be seen from the formula (2), as a numerical aperture NA is larger, a depth of focus is smaller and thereby even a slight displacement of a depth of focus cannot be allowed.
For those reasons, various superresolution methods have been studied in order to increase a depth of focus. A superresolution method is a method in which an optical intensity distribution on an image plane is improved by controlling an illumination optical system, a photomask, or a transmittance or a phase in at a pupil plane of a projection lens system.
So-called off-axis illumination method, which is a method to improve resolution characteristics by optimization of an illumination optical system, has recently drawn attention among various superresolution methods, since it is nearest a reality.
Description will be given on an illumination optical system for an ordinary exposure equipment called a stepper.
In a lithographic process to fabricate a semiconductor device, all the surface of an exposure region on a photomask is required to be illuminated with a uniform intensity in order to control a size of a pattern formed across all the surface of an exposure region of a semiconductor substrate. For this reason, in the ordinary exposure equipment, light emitted from a mercury lamp as a light source is made to pass a cold mirror, an interference filter and the like to have a single wavelength. The light with a single wavelength is guided to a fly's eye lens, which is an optical element for obtaining a uniformity of illumination.
A fly's eye lens is an optical element that comprises a bundle of plural single lenses of the same type aligned in parallel. Each single lens forms a focus point and thereby plural independent point light sources are formed in such a fly's eye lens. When a photomask is illuminated by such a group of point light sources constructed by the point light sources, a uniformity of illumination across the photomask is improved. Sometimes, such a group of point light sources is called a secondary light source, while a mercury lamp is a primary light source.
When light emitted from the mercury lamp is made to pass a fly's eye lens, a state of illumination on a photomask comes to be not affected by a emission state of the mercury lamp as a light source. That is, an illumination state on a photomask is substantially only by a shape and an intensity distribution of a point light source formed by the fly's eye lens, which influences exposure characteristics. This point light source group is called an effective light source from the characteristics.
A method to improve resolution characteristics by controlling a shape of the effective light source is a superresolution method which is generally called off-axis illumination method or an oblique illumination method.
In order to change of a shape of the effective light source, a diaphragm or a filter of one of various forms is generally disposed immediately after a fly's lens. This method is further classified by forms of a diaphragm of the effective light source. For example, an illumination method in which light is intercepted in the middle portion of a diaphragm to form a ring type illumination light source is called annular illumination method, and an illumination method in which a diaphragm having four openings at peripheral four corners is used, is called quadrupole illumination method.
Off-axis illumination method will be described. FIG. 1A is a typical view showing a diaphragm used in an ordinary illumination method which does not employ the off-axis illumination method, FIG. 1B is a typical view showing a main optical system of the exposure equipment as same as FIG. 1A. FIG. 1C is a typical view showing a diaphragm used in illumination method employing the off-axis illumination method and FIG. 1D is a typical view showing a main optical system of the exposure equipment as same as FIG. 1C.
In the ordinary illumination method, as shown in FIG. 1A, a diaphragm 201a which has a circular opening is used. As shown in FIG. 1B, light output from a fly's eye lens 202 is contracted by the diaphragm 201a and projected on a photomask 203 substantially normal to the surface thereof. Then, light is diffracted by the photomask 203 and guided to a projection lens system 204. Light is projected to a semiconductor substrate 205 from the projection lens system 204.
In order that a pattern formed on the photomask 203 is resolved, at least diffracted light of the zero order and plus first order or minus first order are required to be collected from diffracted light. However, in the ordinary illumination method, as shown in FIG. 1B, as a pattern is finer, a diffraction angle is larger and thereby neither of diffracted light of the plus first order and diffracted light of the minus first order is projected into the projection lens system 204.
Therefore, in a fine pattern, light which is projected on the photomask 203 in a direction normal to the surface thereof becomes a noise component which does not contribute to resolution and thus decreases a contrast of a light intensity distribution on an image plane.
On the other hand, in off-axis illumination method, as shown in FIG. 1C, a diaphragm 201b having an opening of a ring like shape is used. As shown in FIG. 1D, incident light is projected on the photomask 203 along a direction oblique to the surface thereof and diffracted light of the zero order and the plus first order or minus first order are projected into the projection lens system 204, so that a majority of illumination light is used for resolution of a pattern.
In such a manner, according to off-axis illumination method, a vertically incident component of illumination light which component does not contribute to resolution is removed and obliquely incident light is effectively utilized, so that improvements on resolution and a depth of focus can be achieved.
However, while off-axis illumination method is effective for formation of a periodic pattern such as produces diffracted light, no effect is exerted on formation of an isolated pattern by which diffraction of light does not occur.
In consideration of such a condition, there is disclosed a photomask in which a fine pattern region not to be resolved is provided in the peripheral region surrounding a main pattern region to be resolved, when an isolated pattern is formed (Japanese Unexamined Patent Publication (Kokai) No. Hei 4-268714). This fine pattern region is hereinafter referred to as an auxiliary pattern region.
A conventional auxiliary pattern region will be described. FIG. 2A is a plan view showing a structure of a conventional auxiliary pattern region and FIG. 2B is a sectional view taken on line A--A of FIG. 2A.
In this case, it is assumed that there is employed an KrF excimer laser exposure equipment in which a ratio of a size of a pattern region on a photomask vs. a size of a pattern on an image plane is 5:1, that is a reducing magnification is one-fifth (1/5), a numerical aperture NA is 0.55 and a coherence factor .sigma. is 0.8. A pattern formed on a surface of a semiconductor substrate, which is an image plane, is an isolated pattern of 0.2 .mu.m.
In a conventional photomask applied with an auxiliary pattern region, as shown in FIGS. 2A and 2B, a light intercepting film 217 made of a chromium film of 70 nm thick and a chromium oxide film of 30 nm thick is formed on a transparent substrate 216 made of quartz. A main pattern region 211 having a width W.sub.1 of 1.00 .mu.m is formed in the light intercepting film 217. This main pattern region 211 is an isolated pattern on an image plane. Auxiliary pattern regions 212 having a width W.sub.2 of 0.5 .mu.m are formed in the light intercepting film 217 with a distance of 1.25 .mu.m from the main pattern region 211 in the left and right side thereof as viewed. In this case, a distance between the main pattern region 211 and an auxiliary region 212 is set so that a pitch of a pattern is equal to a lines and spaces pattern of 0.2 .mu.m. That is, a pitch on an image plane is 0.4 .mu.m and a pitch on a photomask is 2.0 .mu.m. A width W.sub.2 of an auxiliary pattern region 212 is set so that the auxiliary pattern region 212 is not to be transferred.
It is clear that as a width W.sub.2 of an auxiliary pattern region 212 is larger, the effect thereof is larger. However, if the magnitude is in excess of a predetermined value, an auxiliary pattern region 212 itself is transferred onto the semiconductor substrate and sometimes affects a function of the semiconductor device in a wrong manner.
Therefore, an auxiliary pattern region 212 is required to set in such a manner that it may not be transferred by considering various factors, such as a variation of a width W.sub.2 of the auxiliary pattern region caused due to errors in fabrication of a photomask, a variation in an exposure dose when the photomask is used and the like.
If a photomask having such an auxiliary pattern region 212 is used, a depth of focus on an isolated pattern can be larger and the depth of focus is further increased under combination with off-axis illumination method.
In addition to the above mentioned off-axis illumination method, a phase shift mask, which is one of the superresolution method, and which is an improvement on the side of a photomask, is greatly studied.
As a phase shift mask, there is proposed Shibuya-Levenson method, that is a method in which a phase of light transmitted through transparent regions alternately changed by 180 degree in a periodic pattern (Japanese Unexamined Patent Publication (Kokai) No. Sho 57-62052). FIG. 3A is a plan view showing a structure of a Shibuya-Levenson phase shift mask, FIG. 3B is a sectional view taken on line B--B of FIG. 3A and FIG. 3C is a typical diagram showing a amplitude distribution of transmitted light of the phase shift mask.
In the phase shift mask, as shown in FIGS. 3A and 3B, an light intercepting film 227 is formed on a transparent substrate 226 and openings 221 are periodically fabricated in the light intercepting film 227 by selectively removing parts thereof. Transparent films 223 are provided in every other opening 221.
A wavelength of light .lambda. becomes .lambda./n in a material in which it is transmitted, where n is a refractive index of the material. Therefore, there arises a phase difference between light that is transmitted through the air, whose refractive index is about 1, and light that is transmitted through the transparent film 223. In a Shibuya-Levenson phase shift mask, a phase shift is adjusted to be 180 degree by setting a film thickness t of the transparent film 223 to be .lambda./2(n.sub.1 -1). Here, .lambda. indicates a wavelength of exposure light and n.sub.1 indicates a refractive index of the transparent film 223.
When a phase difference between light transmitted through the air and light transmitted through the transparent film 223 is set 180 degree, an amplitude distribution of a transmitted light through a phase shift mask by Shibuya-Levenson method has a distribution in which phases are reversed in every other opening as shown in FIG. 3C. Thereby, in this case, a period of the amplitude distribution is twice as large as that in the case where the phase shift mask is not applied. Therefore, a diffraction angle of a phase shift mask is 1/2 as large as so far and even a pattern which is outside the limit of resolution, the diffraction light is collected into a projection lens.
Since an interference between light beams whose phases are in a reversed relation and light intensity is decreased between adjacent openings, a fine pattern can be separated.
The transparent film 223 is called a phase shifter and a silicon oxide (SiO.sub.2) film is generally used as the material. However, in this case, selective etching is hard to be performed between the transparent film 223 and the transparent substrate 223 made of quartz (SiO.sub.2). For this reason, an etching stopper is required to provide between the phase shifter and the transparent substrate in a Shibuya-Levenson phase shift mask.
However, while with a g-line (wavelength: 436 nm) and an i-line (wavelength: 356 nm) of a mercury lamp, a transmittance of tin oxide and the like is 100% and there is available an etching stopper having light resistance, with KrF excimer laser light (wavelength: 248 nm), there is not available any proper etching stopper.
In such circumstances, a study on a phase shift mask having a structure in which a phase difference is produced has been started for KrF exposure by etching a transparent substrate itself without disposing a phase shifter on a mask. This phase shift mask is disclosed in Japanese Unexamined Patent Publication (Kokai) No. Hei 7-77796. FIG. 4 is a sectional view showing a structure of a conventional phase shift mask described in Japanese Unexamined Patent Publication (Kokai) No. Hei 7-77796. In a Shibuya-Levenson phase shift mask which is fabricated by etching a transparent substrate itself, as shown in the publication, a light intercepting film 237 in which opening patterns 231 on the transparent substrate 236, as shown in FIG. 4, is provided as in the Shibuya-Levenson shift mask shown in FIG. 3. Besides, the transparent substrate 236 is etched and thereby an etching step portion 233 is formed in a position aligning with an opening pattern 231. In this conventional example, the etching step portion 233 works as a phase shifter.
In such a manner, since the phase shift mask shown in FIG. 4 does not use a etching stopper, it can also be applied to KrF exposure and exposure light of a short wavelength, such as ArF excimer laser light. Besides, since a film forming process step for a etching stopper or a phase shifter is not necessary, it has an advantage that occurrence of defects can be decreased.
However, in the case where a pattern is transferred using a phase shift mask which does not require the above mentioned etching stopper, an intensity of light from the etching step potion 233 is decreased on an image plane and thus a serious problem arose that a dimensional difference occurs between adjacent patterns.
In order to solve such a problem, a study has been performed through experiments and simulations and it has been found that the occurrence of a dimensional difference is caused by a change in phase in the vicinity of the etching step portion 233. That is, a phase of transmitted light is not clearly separated into 0 and 180 degrees on both sides at the side wall as a boundary but there is an intermediate phase region in the vicinity of the side wall. In an actual case, since there is light obliquely directed to a mask, reflection and the like occur, as well, at the side wall of the etching step portion 233, so that a more complex phase change arises. A part of light whose phase is changed in a complex manner decreases light intensity from the region.
Another method is proposed in which the side wall of an etching step portion is concealed under an light intercepting film (Japanese Unexamined Patent Publication (Kokai) No. Hei 8-194303). FIG. 5 is a sectional view showing a structure of a conventional phase shift mask described in Japanese Unexamined Patent Publication (Kokai) No. Hei 8-194303. In a phase shift mask described in the publication, a side wall 243a of an etching step portion 243 is spaced from an edge portion of an opening pattern 241 by about 0.1 .mu.m. For this reason, all light whose phase is in disorder is intercepted by an light intercepting film 247 and only light whose phase is changed by 180 degree is made to pass through the opening. This structure can be fabricated by isotropic etching with a buffered hydrofluoric acid or the like after anisotropic etching is applied to the transparent substrate 246 with CHF.sub.3 or the like.
However, the above mentioned various Shibuya-Levenson phase shift masks can only be applied to a closely packed repeated pattern. Then, there are proposed a method in which a Shibuya-Levenson phase shift mask is applied to fabrication of an isolated pattern (Japanese Unexamined Patent Publication (Kokai) No. Hei. 3-15845). A method of an auxiliary pattern method as described in the publication, too, is a method in which a fine pattern region is provided which is not resolved as in a similar manner to a photomask having an ordinary auxiliary pattern region described above. A phase difference between light transmitted through a main pattern region and light transmitted through the auxiliary pattern region is utilized to achieve a phase shift mask effect. Description will be made on a phase shift mask of an auxiliary pattern type having such a structure. FIG. 6A is a plan view showing a structure of a conventional phase shift mask of an auxiliary pattern type and FIG. 6B is a sectional view taken on line C--C of FIG. 6A. In a phase shift mask of a conventional auxiliary pattern type, as shown in FIGS. 6A and 6B, a main pattern region 251 with a width of 1.00 .mu.m is divided in a transparent substrate 256. Auxiliary pattern regions 252 of a width of 0.5 .mu.m are disposed on the left and right sides, as viewed, of the main pattern region 251 in a spaced manner therefrom. Besides, a light intercepting film 257 which has openings at positions above the main pattern region 251 and the auxiliary pattern regions 252 are formed on the transparent substrate 256. A transparent film 253 is formed on the auxiliary pattern regions 252. Thereby, a phase difference arises between light to be transmitted through the main pattern region 251 and light transmitted trough an auxiliary pattern region by 180 degrees.
As a method in which design and fabrication of a mask are simple, there is proposed a half tone method (Japanese Unexamined Patent Publication No. Hei 4-136854). The half tone method has been studied mainly for use in a hole pattern in its first period of development, but later, the method has been found that it has an effect on a general line pattern as well in a combination with off-axis illumination method. In a phase shift mask of a conventional half tone type described in Japanese Unexamined Patent Publication No. Hei 4-136854, a semitransparent film is provided instead of a light intercepting film that is provided for the conventional photomask shown in FIG. 1B. As a material of the semitransparent film, there is used: chromium oxynitride, molybdenum oxynitride silicide or chromium fluoride and a transmittance is commonly in the range of 4 to 10%. In a phase shift mask of a half tone type constructed in such a manner, a phase difference between light transmitted through the semitransparent film and light transmitted through a transparent region in its periphery arises by 180 degrees, and thereby an effect of the phase shift mask can be achieved.
However, in a phase shift mask of a conventional half tone type, there has been a problem in a condition of off-axis illumination that the effect of increase in depth of focus in an isolated pattern cannot be achieved.
For example, in formation of an isolated hole pattern, in a condition of an illumination of a low coherence factor .sigma., the effect of increase in depth of focus more than 50% can be achieved with a half tone mask, but in a condition of annular illumination, there can only available an effect of the same extent as an ordinary mask.
In a photomask having the above-mentioned auxiliary pattern, since there is necessary a fine pattern that is outside the limit of resolution, a problem arises that fabrication of a mask is difficult.
Generally in a photomask having an auxiliary pattern, as a size of the auxiliary pattern is larger, exposure characteristics of a main pattern, such as a focusing characteristic, a depth of focus and the like are improved. However, if a size of the auxiliary pattern is larger, it is transferred. For this reason, a maximum size has been selected possible in the range in which the auxiliary pattern is not transferred.
For example, in the case of a KrF excimer laser exposure equipment (numerical aperture: 0.55, coherence factor .sigma.: 0.8, reducing magnification: 1/5), since the resolution limit is 0.2 .mu.m or less, a pattern of 0.1 .mu.m as an auxiliary pattern, which is half as large as that, is required. With this pattern as an auxiliary pattern, it is a pattern of 0.5 .mu.m on a photomask, which is already lower than the limit at which a pattern can be fabricated in a stable manner in a mask drawing apparatus currently available.
Generally an electron beam pattern generator is used to draw a mask pattern and the resolution limit is on the order of 0.3 .mu.m and a proper exposure dose is changed at a great extent depending on a pattern size.
Therefore, if an exposure dose is adjusted based on a main pattern, an exposure dose is short for requirement of an auxiliary pattern and thereby a size is narrowed by a great margin. When a size of an auxiliary pattern is narrowed in such a manner, an effect of increase in depth of focus cannot sufficiently be achieved. On the other hand, if an exposure dose is adjusted based on an auxiliary pattern, an exposure dose is excessive for requirement of a main pattern, so that an accuracy of a mask size is deteriorated.
Even when a mask pattern is managed to be fabricated in some way or the other, a problem arises in an inspection step that follows the patterning step. That is, if a detection sensitivity is held at a high level in a mask inspecting apparatus, all the auxiliary patterns are detected as a pseudo-defect.
For this reason, while in an actual case, a detection sensitivity of an inspecting apparatus is lowered so that a pseudo-defect may not be detected in operation, as a result, a weak point is brought about that a reliability of a mask is conspicuously degraded.
In light of such a problem, proposed is a phase shift mask in which formation of an auxiliary pattern is easy (Japanese Unexamined Patent Publication (Kokai) No. Hei 5-333524). FIG. 7A is a plan view showing a structure of a phase shift mask described in Japanese Unexamined Patent Publication (Kokai) No. Hei 5-333524 and FIG. 7B is a sectional view taken on line D--D of FIG. 7A. In a phase shit mask pattern of a conventional auxiliary pattern type described in the publication, a conductive film 268 made of indium tin oxide or the like is formed on a transparent substrate 266 made of quartz or the like as shown in FIGS. 7A and 7B. There is provided on the conductive film 268 a light intercepting film 267 comprising a first opening 261 formed in a line like shape and second openings 262 formed on the left and right side thereof, as viewed, equally spaced therefrom. A transparent film 263 is formed on the first opening 261 and the light intercepting film 267. For example, a width of the first opening 261 is 1.5 .mu.m, a width of a second opening 262 is 1.5 .mu.m and a distance between the first opening 261 and a second opening 262 is 1.5 .mu.m. A film thickness of the transparent film 263 in the first opening 261 is set so that a difference in phase of 180 degrees may arise between light transmitted through the first opening 261 and light transmitted through a second opening 262.
In a phase shift mask fabricated in such a manner, light of the zero order transmitted through the first opening 261 and light of the zero order transmitted through a second opening 262 mutually offset by each other and thereby a contrast of an image of the first opening 261 is improved. Furthermore, since a part of light transmitted through a second opening 262 is intercepted on the side wall of the transparent film 263 and furthermore a phase difference arises in part of the light transmitted through the transparent film 263, the second opening 262 is not transferred.
When this conventional phase shift mask and a projection exposure equipment with a numerical aperture NA of 0.45, a coherence factor .sigma. of 0.3, a reducing magnification of 1/5 and exposure light of an i-line (wavelength of 356 nm) was used to perform projection exposure, an isolated space with a width of 0.3 .mu.m was able to be fabricated with a high accuracy.
In this conventional example, since a width of a second opening 262 which plays as an auxiliary pattern region is as large as 1.5 .mu.m, generation of an auxiliary pattern by an electron beam pattern generator is easy. Therefore, reduction in inspection accuracy of an inspecting apparatus is unnecessary in a pattern inspection step.
Furthermore, in the Japanese Unexamined Patent Publication No. Hei 5-333524, an example in which the above mentioned auxiliary pattern method is applied to formation of a hole pattern.
However, in the case of the phase shift mask described in the publication, while in a condition of an illumination of a low coherence .sigma., an effect of large increase in depth of focus is achieved, there is a problem that such a effect of large increase in depth of focus cannot be obtained in a condition of annular illumination.